Orthogonal frequency division multiplexing (OFDM) technique is used in communication systems to transmit digital signals via multiple sub-carriers. The given bandwidth of an OFDM system, shared by all users and the base station, is divided into equal space narrow-bands. Each sub-carrier carries one portion of user data within each narrow-band. In OFDM system, data are divided into frames for transmission. Each frame is further divided into sub-frames or slots and allocated to users and base stations.
FIG. 1 illustrates an exemplary frame structure in TDD mode of OFDM communication. The data are divided into multiple 10 ms long frames for transmission. Each frame is further divided into ten sub-frames with each sub-frame length is 1 ms. For example, Frame n contains 10 sub-frames as illustrated by SF0 to SF9. The ten sub-frames include downlink, uplink sub-frames and special sub-frames. Each sub-frame contains two 0.5 ms long slots. For example, sub-frame SF4 is divided into two slots (i.e., Slot 0 and Slot 1). Each slot consists of seven OFDM blocks and each OFDM block consists of a group of OFDM symbols. The length of each OFDM block is about 71.4 μs. The seven OFDM blocks of Slot 0 are illustrated by blocks 101 to 107. Each OFDM block starts with a cyclic prefix illustrated by the shadow area in FIG. 1.
To prepare for transmission, user data and control information are encapsulated into frames to fit physical layer (PHY) protocol. FIG. 2 shows an exemplary frame structure of an OFDM frame to further illustrate structure of physical layer protocol data units (PHY PDUs). Each frame, such as frame n, is divided into downlink (DL) and uplink (UL) sub-frames separated by transition gaps. Each DL sub-frame consists of only one physical layer (PHY) protocol data unit (PDU). The DL PHY PDU starts with a preamble followed by a frame control header (FCH), and one or more DL bursts. Each UL sub-frame consists of a contention slot for initial ranging, a contention slot for bandwidth requests and one or more PHY PDUs. An UL PHY PDU is made of a preamble followed by an UL burst. As shown in FIG. 2, a DL or UL burst payload carries multiple medium access control messages (MAC Msgs), i.e. MAC PDUs.
In OFDM communication, data of each frame are transmitted block by block. The preamble in each DL or UL PHY PDU consists of an integer number of OFDM symbols. The FCH in the DL PDU is one OFDM symbol long. To form an integer number of OFDM blocks for transmission, the payload of each burst is usually padded by dummy bits. As shown in FIG. 2, packet 211 is padded by dummy bits of pad 221 to form an integer number of OFDM symbols. Similarly, packet 212 and packet 213 are also padded with pad 222 and pad 223, respectively.
Data packetization for OFDM communication according to the existing standard requires the packet boundary being aligned with the OFDM block boundary. No packet is allowed to cross the OFDM block boundary. FIG. 3 shows an example method of packetizing data in a PHY PDU with dummy bits to fit in an integer number of OFDM blocks for transmission. Preamble 301 contains a training sequence inserted into the beginning of the OFDM blocks. The training sequence is one or more OFDM symbols long and filled up OFDM block 340 from OFDM boundary 330 to OFDM boundary 331. Packet 311 is the first packet of the PHY PDU and packetized in OFDM block 341. In order to fill up OFDM block 341 for transmission, pad 321 is inserted after packet 311 until the dummy bits reaches OFDM block boundary 332. Packet 312 is packetized into the next OFDM block and pad 322 is used to align with OFDM block boundary 333. Similarly, Pad 323 is added after packet 313 to reach OFDM block boundary 334.
By using the packetization method illustrated in FIG. 3, each OFDM block only contains data of one packet, and the remaining part after the packet is padded by dummy bits. Since the dummy bits contains no data information, the transmission efficiency decreases with the increasing of the dummy bits.